Quantifying the Excitonic Static Disorder in Organic Semiconductors

Abstract

Organic semiconductors are disordered molecular solids, and as a result, their internal charge generation dynamics, charge transport dynamics, and ultimately, the performance of the optoelectronic devices they constitute, are governed by energetic disorder. This is particularly pertinent for emerging photovoltaic technology where the extractable power is directly dependent on these dynamics. To ascertain how energetic disorder impacts charge generation, exciton transport, charge transport, and the performance of organic semiconductor devices, an accurate approach is first required to measure this critical parameter. In this work, it is shown that the static disorder of an organic semiconductor can be obtained from its photovoltaic external quantum efficiency spectrum at wavelengths near the absorption onset. A detailed methodology is presented, alongside a computational framework, for quantifying the static energetic disorder associated with singlet excitons. Moreover, the authors show that minimizing the limiting effects of optical interference is crucial for achieving high‐accuracy quantifications. Finally, transparent devices are employed to estimate the excitonic static disorder in several technologically relevant organic semiconductor donor–acceptor blends, including the high‐efficiency organic photovoltaic system PM6:Y6.

Comparing Methods of Characterizing Energetic Disorder in Organic Solar Cells

“…On the one hand, the light traverses twice through the absorber layer in FTPS measurements, on the other hand, interference effects can cause differences in the subgap slope. [ 107,108 ] Still, both optical measurements yield relatively low Urbach energies close to thermal energy. This finding coincides well with the model proposed by Kaiser et al., in which they attribute parts of the subgap absorption features to thermal broadening.…”

“…to extract the standard deviation of the Gaussian disorder of the singlet state. [ 108 ] In Figure S10, Supporting Information, we show such a fit to our FTPS data. Contrary to previous observations, it yields a higher disorder with a standard deviation of 41.9 meV for the PBDB‐TF‐T1:BTP.4F‐12 based solar cells than the PffBT4T‐2OD:EH‐IDTBR‐based ones with 35.8 meV.…”

“…[107] Kay et al further observed that the conventional device structure, which is also employed in our samples, is most susceptible to interference effects. [108] Thus, interference could be a possible explanation for apparent Urbach energies E U,app below thermal energy. Note that the increase in apparent Urbach energy E U,app in the low energy regime is not an indicator for deep traps in our samples but simply caused by the limit in the dynamic range of the measurements.…”

Subgap Absorption in Organic Semiconductors

“…This is of particular importance in photovoltaic semiconductor devices, such as solar cells and photodetectors, where the performance is governed by the bandgap energy of the semiconductor in use and can be strongly influenced by energy states within the bandgap . In the case of organic donor/acceptor blend photovoltaic devices, subgap absorption plays a pivotal role in defining photovoltage losses and, ultimately, the losses in the power conversion efficiency, while the spectral line shape of the absorption coefficient has been widely used to infer information about the static disorder, Urbach energy tail states, − CT states, ,− and trap states. − …”

“…At photon energies closer to the optical gap though, the broadening of the subgap line shape is dominated by the energetic disorder. This provides a means to determine energetic disorder associated with the distribution of molecular orbitals as recently proposed by Kay et al 15 In this study, a Gaussian density of states 49 is assigned to the excitonic states in conjunction with Boltzmannlike occupation probability (assuming that the excitonic states are more delocalized compared to the ground state). It was found that the subgap absorption coefficient due to excitons follows…”